First and foremost it depends on how exactly you define Turing-completeness. In simple words you can say that RGB is as Turing-complete as Ethereum. However, if we dig deeper while answering this question, we can find a parallel that comes from physics. Stephen Wolfram, the creator of Mathematica, has been doing a very interesting research and he offered a new approach, new methodology to describe physics. He is the guy who did a lot of research on cellular automata and Complexity Science in the early days. And in his works he shows (mathematically), that you could get a mathematical construction of a hypergraph (graph of a certain type) where you only define simple evolution rules for this hypergraph end up having the emergence of physics as a whole. On this graph the quantum theory & the relativistic theory can emerge, thus proving that very simple rules defined for the graph's evolution (not the step-by-step algorithm!) may create the whole world of a tremendous complexity and richness. If you look closely, you can see that it's very similar to the concept of a cellular automaton. So thus far we see that the same can be applied to RGB, because RGB is basically a hypergraph with certain evolution rules defined locally. And with a kind of smart contracting system like that you can do much more than with Ethereum, where the steps and possible options of your computing are very limited to the predefined set of algorithms that operate the whole state or the system.
We are not 100% sure yet, but maybe RGB potentially is a non-Turing form of computing. By saying that we are not speaking about it being or not being Turing-complete, but about it being a non-Turing form of computing at all, but instead being a cellular automation-based one. So to sum up, RGB is a distributed, partially replicated state machine which is actually the type of a state machine when you don't have a complete state of the system (which is more similar to cellular automaton-based computing that Turing-one).